A sample of 4.92 g of hydrated magnesium sulphate, $MgSO X_{4} \cdot x H X_{2} O$ , is dissolved in water and treated with excess barium chloride solution. The $BaSO X_{4}$ (M = 233 g/mol) precipitated weighs 4.66 g. Using Mg 24, S 32, O 16, H 1, the value of x is

Answer: C

Each mole of the salt gives one mole of

BaSO X_{4}

, so moles of salt

= 4.66/233 = 0.0200

. Molar mass of the hydrate

= 4.92/0.0200 = 246

g/mol. Anhydrous

MgSO X_{4}

is 120, so

18 x = 126

and

x = 7